Many heat exchanger systems are known which function with liquid and gaseous media and more such systems are constantly being developed for new industrial applications. As novel precision manufacturing and control components are being devised, they create needs for new systems that use heat exchangers to establish and/or maintain operating temperature levels within particular limits in those components while meeting different criteria as to resolution, heat flux and the like. There are also concurrent demands for thermal efficiency and low cost operation in such systems that must be met.
In some modern industrial applications, for example, the products being processed must be brought to particular temperature levels to accomplish specific functions. After this the temperature level may have to be changed so that a different function can be carried out. In these modern systems, efficiency, temperature control and freedom from pollution are necessary prerequisites. As is usually the case, low purchase and continued ownership costs, are virtually necessary for these systems. Heating and cooling systems must often respond quickly when operating temperatures are changed in order to minimize expensive wait times in the production process. They must also operate faultlessly over long time spans with little down time. Nevertheless, for cost reasons the new designs should also be adaptable with little modification to different heating, cooling, temperature range and heat load situations.
Semiconductor fabricating facilities, for example, provide good illustrations of the demands often found in modern heat exchange applications. Semiconductor elements are generally initially processed as multiple small closely distributed local areas of semiconductor wafers which are subsequently subdivided into the individual components. As wafer sizes have increased with time, e.g. to 300 mm in diameter as is presently the case, demands for efficient and reliable operation of their support components has become ever mole important. This is because the cost of each wafer has increased in proportion to its area. A 300 mm wafer is 2.25 times the area of its predecessor, the 200 mm diameter wafer. During manufacture the wafers are positioned at or transported between different “cluster” tools at which they are heated or cooled to different levels, for chemical, radio-frequency radiation and other steps. Each of these subprocesses is carried out for a limited time, usually in a controlled temperature range. The temperature control systems employed use different heat and refrigeration sources, and the heating and cooling loads and the temperatures involved are widely different in some respects, but quite similar in others.
For these reasons, it is often possible to utilize a single given source of thermal control to provide controlled temperature environments for carrying out different functions at cluster tools. Also, it is common in these and other applications to try to utilize environmentally safe refrigerant systems and efficient and stable heat exchange systems together with electronic controls to establish and maintain precise temperatures for given times. An efficient and versatile heat exchanger is vital to temperature control for these and other systems. If the heat exchanger can accommodate or be adaptable to the use of different fluid media at different temperature levels, and use different energy sources, new and cost advantageous temperature control units (TCU), as these units are termed, can possibly be provided. Such units can potentially meet the ever more stringent demands for cost performance and efficiency.
The typical TCU uses a heat transfer fluid, typically either a mixture of water and ethylene glycol or a perfluorinated fluid (e.g. Galden), that is pumped in a closed circuit between the TCU and the load to be controlled at a desired temperature. This fluid is heated or cooled as dictated by thermal conditions at the load. The pump driving the flow of heat transfer fluid used is part of the TCU, as is any reservoir of heat transfer fluid, and any control subsystem needed.
Heat exchangers are needed to effect the heating and cooling within the TCU. Cooling is most typically supported by a vapor-cycle refrigerator in evaporating a condensed fluid to vapor in conventional vapor-cycle manner. The heat exchanger effects the transfer of heat from the heat transfer fluid to the refrigerant while maintaining physical isolation between the fluids. When the heat transfer fluid needs to be heated sometimes the same passage that is used to boil refrigerant during the cooling process is fed a supply of hot compressed gaseous refrigerant from the output of the vapor-cycle compressor. Alternatively, heat required can be supplied by an electrical heater in another heat exchanger to either supplant the hot gas heat or said electrical heater and its heat exchanger is sometimes used as the entire heat source.
Heretofore, the heat exchangers described above have been designed and built in two different assembly configurations in various TCUs.
1. When a fairly leisurely rate of temperature change can be accepted, the TCU is designed with a large reservoir of heat transfer fluid in which reservoir are immersed any electrical heaters as well as a long tube within which is passed the evaporating fluid or hot gas to cool or heat as discussed above.
2. The other common configuration uses separate discrete heat exchangers to perform the two functions involving the refrigerant in one case and the electrical heater in the other.
The first configuration has the advantage of low cost and a reasonably small volume. Since a large fluid reservoir is often needed it requires little cost to design immersed tubes and electric heaters within said reservoir. This configuration has the disadvantage of slow response along with a reduced level of efficiency when compared with the configuration using discrete heat exchangers. This is because the velocity within the large reservoirs is, of necessity, low and the characteristic distances within the heat transfer passages large. Thus the heat transfer coefficient between the fluid and the wall of the heat exchanger(s) is low. This results in large temperature differences being needed to transfer the required amount of heat. The heating/cooling TCU thus must operate at higher maximum temperatures when heating and lower minimum temperatures when cooling. This results in a much lower thermal system efficiency for the TCU than is possible with more optimally designed heat exchangers. Also, as noted, the system is sluggish in response to changing temperature needs. This slow response is also a result of the inefficient heat exchange used.
The disadvantage of the second configuration is twofold. The volume required to direct a nest of piping to and from each of the discrete heat exchangers involving flow of refrigerants, heat-transfer fluid and electrical power is generally large, and the cost of such piping, together with the required fittings, is often an excessive part of the overall cost. Also, conventional heat exchangers that are at once efficient and capable of withstanding high internal refrigerant pressure over a wide range of temperature are costly in themselves.